Hi there. I refer to tcgplayer when I suspect a card might be counterfeit. This link is a good guide.
Additionally, fake cards can have subtle odors and even different opacity when compared to authentic cards, though this is harder to detect.
When I'm holding an authentic Pokémon card, there is almost always a different texture compared to a counterfeit, but as the quality of fake cards improves, there may soon be a day when they're seemingly identical.
As a last resort, counterfeits can sometimes be weighed on a digital pocket scale. They're often heavier than authentic cards.